Objectives The periodical nitrogen absorption and fertilizer use efficiency by rice, as well as the behaviors of fertilizer nitrogen were studied under the Chinese milk vetch and rice straw returning to the field and reducing 40% of conventional nitrogen fertilizer input, to provide a theoretical basis for the scientific fertilization in rice cultivation.

Methods The field experiment was started in 2017, located in Chizhou City, Anhui Province. Five treatments were set up: winter fallow + conventional N application (100% N); winter fallow + 40% N reduction (60% N); winter fallow + rice straw return + 40% N reduction (60% NS); winter Chinese milk vetch + 40% N reduction (60% NMv), and winter Chinese milk vetch + rice straw return + 40% N reduction (60% NMvS). On the 5, 10, 20, 40, 60, 80, 100, and 114 days after transplanting, rice plant samples were collected for the determination of biomass and nitrogen content. The rice yields were investigated at harvest. A micro-plot (50 cm in diameter) was arranged in each treatment plot for the determination of fertilizer nitrogen behavior. The ¹⁵N-labled urea was used in the micro-plot experiment to replace conventional urea, all the other procedure was exactly the same as the fertilizer treatment plot.

Results Reducing chemical N fertilizer by 40% (60%N treatment) significantly decreased the N absorption amount and rate after the tillering stage. The N absorption rate of rice in 60%NS treatment was the lowest during seedling−tillering stage, but gradually increased after the tillering stage, and reached the highest during the grain-filling stage. The N absorption rate in 60%NMv treatment was higher than 60%N but lower than 100%N treatment during seedling−jointing stage, and exceeding 100%N treatment after jointing stage. The combined application of rice straw and milk vetch (60%NMvS treatment) maintained relatively high N absorption rate across whole growing stages of rice, and the total N absorption in this treatments was significantly higher than all the other treatments. The ¹⁵N tracer experiment showed that 60%NS treatment enhanced rice N uptake from soil, 60%NMv treatment improved rice N uptake from both fertilizer and soil. 60%NMvS treatment was recorded similar fertilizer N uptake to that of 60%NMv treatment, but significantly higher soil N uptake. The 60%NMvS and 60%NMv treatments exhibited higher N fertilizer use efficiency than the left treatments; 60%NS treatment was recorded the highest soil N residue rate, followed by the 60%NMvS treatment. 60%NMvS treatment was recorded the lowest N loss rate, with 60%NS and 60%NMv treatments having the second lowest.

Conclusions Incorporating Chinese milk vetch into rice fields is beneficial for enhancing N absorption during the early to mid-growth stages, while returning rice straw promotes N absorption during the later stages. The combined incorporating of Chinese milk vetch and rice straw could guarantee the rapid and high N absorption during whole rice growth season, under the condition of reducing 40% of chemical fertilizer input. Incorporation of Chinese milk vetch is effective in improving N fertilizer use efficiency, while returning rice straw benefits soil nitrogen retention. Both practices help reduce N loss. Therefore, combined incorporation of Chinese milk vetch and returning rice straw leads to improved N absorption amount, reduced fertilizer N loss, and enhanced soil N residue.